1. Field of the Invention
The present invention relates to a focused ion beam (FIB) system and, more particularly, to extraction of an ion beam from an emitter.
2. Description of the Related Art
FIB (focused ion beam) systems for processing of materials using focused ion beams arising from a metal ion source, such as Ga, and capable of functioning as a scanning ion microscope (for example, a focused ion beam is directed to a material while scanning the beam in two dimensions, secondary ions originating from the material being detected, and an SIM (scanning ion microscope) image being produced) have enjoyed wide acceptance.
One example of the ion optical system of such an FIB system is shown in FIG. 4, where an ion illumination system has a microscope column 1 whose interior is maintained as a high vacuum. An emitter 2 serves as an ion source. An extraction electrode 3 is used to extract an ion beam. An acceleration electrode 4 accelerates the ion beam to a desired energy. A condenser lens electrode 5 cooperates with the extraction electrode 3 and the acceleration electrode 4 in acting as a condenser lens 5a. The condenser lens 5a adjusts the current of the ion beam to a desired current value and is of the electrostatic type. An aperture 6 limits the angular width of the ion beam. Deflection electrodes 7 deflect and scan the ion beam in two dimensions. An objective lens 8 focuses the ion beam onto a specimen 9 to be processed or imaged by scanning ion microscopy. For example, the objective lens 8 is an electrostatic type and made up of three electrodes. The specimen 9 is placed on a specimen stage 10 capable of adjusting the position and the tilt of the specimen. These components and the specimen 9 are placed within a specimen chamber 11. Power supplies (not shown) and a control unit (not shown) are connected with all of the emitter 2, the various electrodes, such as the extraction electrode 3, the various lenses, such as the condenser lens 5a, and the specimen stage 10 to operate them. A detector (not shown) for detecting signals, such as secondary ions, produced from the specimen 9, a processing circuit (not shown) for processing the output signal from the detector, a display device (not shown) for displaying SIM images, and other devices are connected with the control unit and power supplies.
When the material or specimen 9 is processed with the ion beam or an SIM image is generated by this FIB system, it is desired that the smallest possible diameter of the ion beam be obtained and that the ion beam current be sufficiently stable. Therefore, in a general FIB system, a suppressor electrode S is added between the emitter 2 and the extraction electrode 3 as shown in FIG. 5 to regulate the emission current from the emitter 2. The insertion of the suppressor electrode S has many advantages other than its capability to easily regulate the emission current or the ion beam current. For example, the extraction voltage can be maintained constant. In addition, if a voltage applied to the suppressor electrode S is varied for regulation, the ion beam is hardly affected. That is, the hit position, the beam diameter, and other factors associated with the ion beam are hardly affected.
The addition of this suppressor electrode S inevitably increases the extraction voltage. For instance, where extraction is achieved at 4 to 5 kV, if the suppressor electrode S is added, the extraction voltage is increased to about 9 to 12 kV. Where the extraction voltage is increased in this way, the beam diameter is undesirably increased as shown in FIG. 6.
In recent years, specimen feature sizes have become smaller and smaller. Therefore, the need for finer ion beams has become more urgent. Accordingly, it has been considered to omit the suppressor electrode S. If so, the emission current is regulated by the voltage applied to the extraction electrode 3. In particular, the voltage applied to the extraction electrode 3 is so controlled that the difference between the presently measured current and the target current becomes null.
However, experiment has shown that this method has the following disadvantage. If the amount of feedback is not appropriate, the control cannot be provided with good response or oscillation takes place. To avoid such a phenomenon, the emitter should be maintained in a constant state. For this purpose, a flashing operation is frequently performed by temporarily energizing a heater mounted close to the emitter to heat the emitter, thereby removing contaminants from the surface of the emitter. In this way, the emitter is refreshed at all times. However, frequent flashing lowers the throughput of the instrument and shortens the life of the emitter.
It is an object of the present invention to provide an FIB system capable of producing finer ion beams and sufficiently stable emissions.
A first embodiment of the present invention provides a focused ion beam (FIB) system having an emitter acting as an ion beam source, an extraction electrode for extracting an ion beam from the emitter, an acceleration electrode for imparting energy to the extracted ion beam, at least one lens for focusing the ion beam, and a specimen stage on which a specimen to be irradiated with the focused ion beam is placed. The FIB system is characterized by controlling emission current. The emission current is controlled by adjusting the voltage applied to the extraction electrode. The voltage applied to the extraction electrode is set as a function of the target emission current, the currently measured emission current, and the accumulated emission time from the last flashing.
A second embodiment of the present invention provides a focused ion beam (FIB) system that also has an emitter acting as an ion beam source, an extraction electrode for extracting an ion beam from the emitter, an acceleration electrode for imparting energy to the extracted ion beam, at least one lens for focusing the ion beam, and a specimen stage on which a specimen to be irradiated with the focused ion beam is placed. The FIB system is characterized by controlling emission current substantially as described for the first embodiment. The voltage applied to the extraction electrode is set as a function of target emission current, currently measured emission current, accumulated emission pause time from the last flashing, and accumulated halt time since the last flashing.
A third embodiment of the present invention provides an FIB system which is based on the first or second embodiment described above and further characterized in that a flag is used to inhibit change of the voltage applied to the extraction electrode described above.
A fourth embodiment of the present invention provides an FIB system which is based on any one of the first, second, or third embodiments described above and is further characterized in that the voltage applied to the extraction electrode described above is controlled by computer software.
Other objects and features of the invention will appear in the course of the description thereof, which follows.